In modern vehicles, radar systems are increasingly used, i.e. for sensing neighboring objects/targets (including other vehicles), for lane changing, collision avoidance and other driver assist functions.
Unambiguous discrimination in radar systems with respect to angle, Doppler and range remains an area of investigation. Angular resolution is physically limited by the total antenna array size. The known virtual Multiple-Input-Multiple-Output (MIMO) concept provides better angular resolution with the same number of antenna elements with respect to their phased array counterpart. The utilization of sparse arrays and orthogonal signals leads to a virtually filled array in the processing unit. Achieving orthogonality with respect to the transmit signals has been extensively discussed in the prior art.
A simple approach of enabling the virtual MIMO scheme is the Time-Division-Multiplex (TDM) MIMO principle; see D. Zoeke and A. Ziroff, “Phase migration effect in moving target localization using switched MIMO arrays,” Proceedings on the 12th European Radar Conference, 2015. In this concept, the independence of waveform has been achieved by switching the transmit Tx and the receive Rx channels, such that all combinations of Rx and Tx have taken place within the coherent processing interval (CPI). Analysis of this approach leads to the understanding that the TDM MIMO concept is effectively a linear phase center motion within the antenna structure. Due to the linear phase center motion (PCM), the target angle is coded into frequency. However, if there is an additional target motion which distorts the PCM, a strong angle-Doppler coupling appears and has to be addressed. In the Zoeke and Ziroff paper, this problem is addressed by using one moving and one stationary phase. The stationary phase center (PC) is used for Doppler compensation. A problem is that this approach works only as long as no other target appears at the same range bin with a different Doppler shift. The result of the latter is that ghost targets are present within the range-angle-Doppler map.
Moreover, in the approach disclosed in the Zoeke and Ziroff paper, within an antenna array of receivers and transmitters, just one transmitter and one receiver are switched on simultaneously and the phase center is taken to discrete positions within the array. The switching schemes used are called mixed, interleaved, stacked and stacked multiple, and all lead to a linear PCM. All of such schemes are coding the angle, and exploiting the orthogonality in angle, for the virtual MIMO approach, but all are suffering from phase variations which are caused by motion target of the target. The switching scheme itself can be interpreted as a kind of phase center motion for a single trajectory. Accordingly, with the disclosed technique, only a single trajectory is used; and the trajectories presented all suffer from target Doppler shifts and therefore are not orthogonal in angle and Doppler simultaneously as it is in the case of the present invention.
In Y.-B. G. Shiwen Yang and P. K. Tan, “Linear antenna arrays with bidirectional phase center motion”, IEEE Transactions on Antennas and Propagation, vol. 53, no. 5, pp. 1829-1835, May 2005, bidirectional linear phase center motion in a linear array is presented. The authors introduce Doppler shifts affected by phase center motion. Those Doppler shifts are used to shift the power of the radiation pattern side lobes into another spectrum. The Doppler shifting of side lobes is caused by the angle dependency of an effective phase center trajectory which is seen by a target in the far field.
In Y. B. G. Shiwen Yang and A. Qing, “Sideband suppression in time-modulated linear arrays by the differential evolution algorithm”, IEEE Antennas and Wireless Propagation Letters, vol. 1, pp. 173-175, November 2002 and J. Guo, S. Yang, S.-W. Qu, J. Hu, and Z. Nie, “A study on linear frequency modulation signal transmission by 4-d antenna arrays”, IEEE Transactions on Antennas and Propagation, vol. 63, no. 12, pp. 5409-5416, December 2015, further studies were done with respect to applying a radiation pattern design algorithm and other forms of switching of the antenna center other than the linear approach. However, the disclosed techniques all involve a single-trajectory approach and are not concerned with orthogonality in trajectory.
In D. R. Fuhrmann and G. S. Antonio, “Transmit beamforming for mimo radar systems using partial signal correlation”, 2004, a model is provided that uses only correlation between antenna elements, i.e. only space correlation, with no time correlation.